...Why this impact apparently emitted so much light?
I get that the asteroid probably had a LOT of kinetic energy, but isn't it only in "Hollywood physics" that when two inert things collide you get a fiery explosion? .... and I'm even more surprised as it took place in a vacuum where my limetd understanding of conventional physics says fire cant happen...
Explosions are not, in general, anything like "fire" (fuel burning in air by slowly mixing with it). Explosions are the sudden conversion of energy in a compact mass into heat, and the sudden expansion of that same, now very hot, mass. All of the energy in a chemical explosion is already present in the explosive - be it a mixture like gun powder, or high energy chemical molecules (TNT), or a high velocity object. Otherwise guns wouldn't work (cartridges are essentially sealed), torpedoes wouldn't work (explosions under the sea?), etc,. etc.
An asteroid or meteoroid hitting the Moon would be travelling at least at 11,000 m/sec (could be as high as 70,000 m/sec), which gives it a kinetic energy equal to at least 60 million J per kg, or 15 times (could be 500 times) the energy of an equal mass of TNT. The result is a much hotter and brighter explosion when it hits something than an equally energetic mass of high explosive.